Tbh, I'm undecided as to whether I want Fire, Blizzard, and Thunder IVs at all. Ideally, I'd revamp all 12 animations (Fire I, II, III; Blizzard I, II, III; Thunder I, II, III) slightly, so that they appear stronger in proportion to their bonus effectiveness given by Enochian. (Thunder IV's effect, clearly the most aesthetically pleasing and "BLM-like", would replace Thunder III's, which now seems to me to be a bit too easily exploited, and therefore an obligatory gimmick.) At that point, yes, your primary elemental casts in Enochian would refresh their respective elemental attunements (AF/UI). Without that, I'm unsure. I actually like having to weave in Fire I alongside Fire IVs; without that need it just feels like I may as well have two separate keyboards, one for regular play and one for Enochian—they'd feel too disconnected—and I'd rather slightly increase on that merge.
As for why I wanted so many Thunder skills, the main point was to try to develop greater symmetry between all three Elements. Fire and Ice will rule your invocation/evocation or your mana feed/burst, but Thunder should be more than just a DoT in varying levels. I still want to flesh out what makes Lightning unique beyond just its ability to be charged up for greater effect, but that will probably have to wait until I've decided what to do with Shattering, the new Ice mechanic.
The 4 different Thunder casts should each have situational usage even within Enochian. Because Enochian allows for a third charge, or a forth charge rank, so to speak, your consuming Thundercloud stacks two at a time become more efficient; this benefits all four spells quite well.
Depending on the number of (Electrified) enemies, Thunder IV may be your strongest nuke (if not the strongest in the game), besides perhaps a well ramped Enochian Flare. But you wouldn't generally forgo reapplication or spreading of Electrified just to spam it unless you couldn't make use of the DoT. Moreover, over time it will still deal less damage than the fire line.
That bit is actually due to the movement of Fire damage natively to partially periodic damage. Because each brief DoT inflicted is added onto the next fire spell's damage, it ramps up in a tapering but otherwise infinite fashion, limited only by its spell speed—which, with the adjusted Firestarter, it improves upon. That gives BLM an even higher damage ceiling, but also a higher skill cap. It may be beneficial to delay nuke hits briefly as to create longer DoTs on multiple enemies, which will in turn increase your speed so that you can maintain a rate of fire that consistently ramps on top of itself. Your final Fire spell in a series of crits before needing to refresh for mana should hit incredibly hard. In other words, you might not open with a 5-digit Fire spell; but you wouldn't even necessarily need to crit to see it by the end of a burst swing.
I had intended Scathe to be a bit utility-purposed, but still worthwhile enough that you might consume a GCD and relevant resources on it at certain rotational situations (e.g. couldn't get enough crits in your Fire line), rather than being used only for its safety and movement provided. It's especially fresh, though, and only one of a few ideas towards that purpose. I've also considered making Fire, Ice, and Lightning procs directly allow it to be oGCD, rather than just rewarding its crits with a free second cast, though I slightly prefer this for now; I like a bit of chance, especially now that Firestarter and Thundercloud are less random, and this is a sort that seems more rewarding to get than punitive to not get.
Personally I'd just like an option in the character menu for "spell line suffixes: <alpha> <numeric>". Can leave the default at numeric though. I'd like the option to go "Fire-Fira-Firaga-Firaja" instead, but I'm not sure I'd actually bother with it, and would probably still just write them "F1, F2, F3, F4."